ASBESTOS

Asbestos is a general name applied to many silicate minerals that readily separate into long, thin, flexible fibers. The asbestos minerals that are suitable for industrial uses include serpentine asbestos, amphibole asbestos and brucite asbestos. Serpentine asbestos, also known as chrysotile, possesses good cleavability, a high mechanical strength, a good resistance to heat and a high electric insulation, as well as such properties as filterability, film-forming beating ability, magnetism and electrokinetic potetial. It is widely used in the construction, machine-building, petroleum, chemical metallurgy, electronics and transportation industries, as well as in the military and space industries. At present, owing to the requirements of environmental protection, many countries are developing substitutes of asbestos. It is estimated that no major breakthrough will be made in the near future. Therefore, asbestos minerals will still occupy an important position in the national economy.

Amphibolite asbestos includes crocidolite asbestos (blue asbestos), anthophyllite asbestos, tremolite-actinolite asbestos and amosite asbestos. China's crocidolite asbestos is principally magnesioriebeckite, which is highly resistant to acids and has special filterability and so is widely used as gas and liquid cleaning material in the industrial production. It is the only natural fiber material for purifying poisonous gas and so is one of important strategic minerals.

Chrysotile, actinolite-amphibole asbestos and crocidolite asbestos account for 99.2%, 0.7% and 0.1% of the retained reserves of China's asbestos minerals respectively. This section mainly introduces China's chrysotile and some actinolite-amphibole asbestos and brucite-serpentine asbestos, which are called by a joint name-asbestos minerals.

As early as more than 900 B.C., the Chinese people had woven asbestos into cloth called "huowan" (fireproof). At the beginning of the 13th century asbestos was mined sporadically. In 1913 the Xiaozhushan asbestos deposit, Jinzhou, Liaoning, began mining. In 1949 China's asbestos production was only 550 tons. Since 1949 China's asbestos industry has undergone the stages of high-speed, rapid and steady development and thus the production has increased progressively. In 1997 there were 103 asbestos mining and beneficiation enterprises in China, which produced 437,000 tons of asbestos per year.

4.16.1 Resources

4.16.1.1 Resources and Reserves

China has abundant asbestos mineral resources. By the end of 1997 China had explored 45 asbestos occurrences with retained categories B+C+D asbestos reserves of 90.44 million tons, ranking third in the world.

Among the retained reserves categories B+C reserves take up 35%. For large and medium-sized deposits, exploration or detailed reconnaissance has been basically carried out, and for a few particular medium-sized deposits that have been mined, only reconnaissance has been conducted. Among the occurrences with retained reserves, 25 (5 large, 5 medium-sized and 15 small) have been mined, which are mainly distributed in Qinghai, Sichuan, Xinjiang, Shaanxi, Hebei, Shanxi, Yunnan, Jiangxi and Anhui. They contribute a total of more than 63 million tons of retained asbestos reserves, accounting for 70% of the total retained reserves, which may meet the demands of China's asbestos industry in 2010. There are 16 occurrences (1 large, 2 medium-sized and 13 small) that can be selectively mined. They have a total of more than 26 million of retained asbestos reserves, making up about 29% of the total retained reserves, and are mainly distributed in Qinghai, Sichuan, Beijing, Heilongjiang, Shandong, Yunnan, Shaanxi, Xinjiang and Guangxi. Except the West Mine of the Mangnai asbestos mine(formerly known as Mangya asbestos mine)which is large in size and can be used as a reserve mine of the Mangnai asbestos mine, the others are mostly small mines that were mined but now have stopped mining, and there are few reserve occurrences that can be used for the construction of new mines.

In addition, there are 11 crocidolite asbestos occurrences with a total of 44,500 tons of retained reserves, distributed in Yunnan, Henan, Shaanxi and Sichuan. The reserves are mainly concentrated in Yunnan, which contributes 85% of the national total crocidolite asbestos reserves. In most of these occurrences exploration or detailed reconnaissance have been carried out. Of the retained reserves, categories B+C reserves make up 69% of the total reserves. Four occurrences have been mined but now three of them have stopped production except the Gaofengsi crocidolite asbestos deposit at Yao'an Yunnan.

4.16.1.2 Characteristics of Resources

China's asbestos deposits are distributed in 15 provinces (municipalities and autonomous regions). The distribution of 13 large and medium-sized asbestos deposits with retained reserves is shown in Fig. 4.16.1. Ninety-nine percent of China's asbestos retained reserves are distributed in western China, and the retained asbestos reserves in northwestern China take up 79% of China's total asbestos reserves. In this region, Qinghai Province possesses most asbestos reserves, accounting for 64% of the national total. The Mangnai deposit in northwestern Qinghai is China's largest asbestos deposit. Its East and West mines total more than 40 million tons of retained asbestos reserves, accounting for 48% of the national total. The northwestern part of the Mangnai deposit adjoins four small (or medium-sized) deposits in Ruoqiang and Qiemo counties, Xinjiang. These places, which together contribute over 50 million tons of retained asbestos reserves, is China's most important asbestos-containing area. At Heicigou, Xiaobabao and Shuangchagou, Qilian County, on the Qinghai-Gansu border there occur five small to large asbestos occurrences with more than 16 million tons of retained asbestos reserve; at Da'an, Ningqiang County, and Jianchaling, Lueyang County, southwestern Shaanxi Province, there is one large or medium-sized deposit with over 10 million tons of retained asbestos reserves. The above-mentioned two areas are also China's major asbestos-containing ones. In addition, there are small deposits at Yushugou, Toksun County, Xinjiang, and at Annanba, Aksay, Gansu. Abundant asbestos resources make the northwestern region become China's most important base where asbestos production develops most rapidly.

The retained asbestos reserves in southwestern China account for 20% of the national total, mainly distributed in Shimian County southwest of Ya'an City, western Sichuan. This county was established in June 1951 when the Xichang People's Government separated the adjacent areas of original asbestos-rich Hanyuan, Yuxi and Mianning counties from the three counties to establish Shimian County in order to speed up the development of the asbestos industry. With a total of nearly more than 16 million tons of retained asbestos reserves, the North and South mines in Shimian County are one of the major asbestos-containing areas and major asbestos-producing areas, and so have certain development prospects. The medium-sized Wudaoniupeng asbestos deposit in Kangding County northwest of Shimian County had been mined but the mining was stopped in 1983. Besides, small asbestos deposits occur sporadically at Shishan of Wuding County in northern Yunnan Province, Dayutang of Xinping County, Mojiang-Yuanjiang and Midi of Honghe in southern Yunnan, and Gangpo of D¨ºq¨ºn County in northwestern Yunnan; they have been mostly mined.

Asbestos deposits in eastern China are principally small in size and have been mostly mined. They have only nearly 2 million of explored reserves. Through a few decades of production, most mines have entered the late stage of mining. Among the original three medium-sized mines, the Jinzhou mine in Dalian City and Dapingfang mine in Chaoyang County, Liaoning Province, which were developed earlier, have been closed and the remaining Yanmeidong mine, Laiyuan County, Hebei Province, has only 150,000 tons of retained asbestos reserves. Others are all small mines scattered in various areas, including the Jinzhucun mine, Jingjing County, Hebei Province; the Beitagou mine, Yilian County, Heilongjiang Province; Xinmin mine, Fanshan County, Shanxi Province; the Hongshuigou mine of Mentougou and Shiyao mine of Yanqing, Beijing; the Suoluoshu mine, Rizhao, Shandong Province; the Honglong mine, Ningguo County, Dishuiya mine, Tongcheng County, and Lihe mine, Shucheng County, Anhui Province; Dabeng mine, Xinyi County, and Matang mine, Yunfu City, Guangdong Province; as well as by-product asbestos deposits in serpentinite ore districts at Zhonggangban and Shizishan of Zhiyuan, Yiyang County, Jiangxi Province, and Tanggoushan and Lindong (Beidi), Rongshui County, Guangxi Province. Most of them have been mined and some of them have stopped mining. Generally, the asbestos mines in eastern China have limited minable reserves left and some of them will be closed in three to five years. Although the resource potential might be tapped in some old mines, their reserves are very difficult to increase by a big margin. Therefore, eastern China has limited prospects for expanding the asbestos development in the future.

Except a very small number of medium-sized and small asbestos deposits in Kangding, Sichuan, and Ningguo, Anhui, which are actinolite and amphobole asbestos deposits, the great majority of China's asbestos deposits are serpentine asbestos (chrysotile) deposits. According to the industrial classification, China's chrysotile deposits may be classified into the magnesian ultrabasic rock type (or ultrabasic type for short), including brucite-serpentine asbestos deposits, and the magnesian carbonate rock type (or carbonate type for short, and also called the dolomite type). According to their origin ultrabasic type deposits may be further divided into meteoric water-hydrothermal alteration subtype (e.g. the asbestos deposit in Sichuan), migmatitized hydrothermal alteration subtype (e.g. the Rizhao deposit in Shandong), groundwater mixing-hydrothermal alteration subtype (e.g. the Mangnai deposit-formerly known as Mangya deposit-in Qinghai) and postmagmatic hydrothermal alteration

Fig. 4.16.1Distribution of asbestos occurrences of China
1. Arpa, Qiemo, Xinjiang; 2. West mine of the Mangnai asbestos deposit, Mangnai Township, Qinghai; 3. East mine of the Mangnai asbestos deposit, Mangnai Township, Qinghai; 4. Heicigou, Qilian County, Qinghai; 5. No. II mine of the Xiaobabao deposit, Qilian County, Qinghai; 6. East-central mine of the Shuangchagou deposit, Qilian County, Qinghai; 7. Yanmeidong, Laiyuan County, Hebei; 8. Jianchaling, Lueyang County, Shaanxi; 9. Heimuling, Da'an Ningqiang County, Shaanxi; 10. Wudaoniupeng (actinolite-tremolite asbestos), Kangding County, Sichuan; 11. North mine of the Shimian deposit, Shimian County, Sichuan; 12. South mine of the Shimian deposit, Shimian County, Sichuan; 13. Mojiang-Yuanjiang, Yunnan.

subtype (e.g. the Qilian deposit in Qinghai). Carbonate type deposits may be further divided into postmagmatic hydrothermal alteration subtype (e.g. the Qilian deposit in Qinghai), contact-metasomatic wet-skarn hydrothermal alteration subtype (e.g. the Laiyuan deposit in Hebei) and migmatitized hydrothermal alteration subtype (e.g. the Fangshan deposit in Shanxi).

Ultrabasic type asbestos deposits were formed chiefly in the Precambrian and Hercynian and less commonly in the Caledonian and Yanshanian. Precambrian deposits are mainly distributed in Sichuan and Shaanxi, with those occurring in the front of the paleovolcanic island arc; Hercynian deposits are distributed in the Altun, West Kunlun and Tianshan mountains; Yanshanian deposits are found in Yunnan. The deposits occur in belts and deposits of the same metallogenic epoch are concentrated in terms of space. One metallogenic belt only belongs to one metallogenic epoch.

Carbonate type deposits are closely related in time and space, chiefly distributed on margins or in the interior of the North China paraplatform and Yangtze paraplatform. These platforms have Precambrian basement. In the North China paraplatform the country rocks of this type of deposit are the Cryptozoic Eon, mainly the Lower Sinian and Archean; in the Yangtze paraplatform the country rocks of the deposits are chiefly Precambrian and late Sinian, i.e. Phenorozoic, strata. No matter what ages may be, the country rocks of this type of deposit are all related to Hercynian and Yanshanian acid and basic rocks, and only deposits of metamorphic origin are related to Archean acid rocks.

The metallogenic characteristics of the ultrabasic type deposits are as follows: the deposits mostly occur in deep-plunging, older eugeosynclinal fold belts and are hosted in ophiolites of particular geological periods; the deposits show metallogenic specialization, i.e. they only occur in ultrabasic rocks of particular periods, and the size of an intrusion has a certain influence on the size of a deposit; the parent rocks favorable for mineralization are a magnesian ultrabasic series and a magnesioferruginous ultrabasic series, and the rocks with Mg/Fe>7 are favorable for mineralization and the rocks with Mg/Fe>10 are mostly harzburgite and peridotite; the formation of the deposits went through a multi-stage, complex process; serpentinization generally occurred in two stages, i.e. the early stage of universal weak alteration and the late stage of replacement and recrystallization; the concentration of asbestos veins is related to the degree of development of secondary fissures and the network veins are high-grade; the Precambrian and Hercynian are the most important metallogenic epochs and there are also Yanshanian medium-sized deposits; the orebodies are simple in shape and mostly occur as beds, lenses and veins; the deposits are commonly characterized by a large size, large number and relatively high grade; the deposits are mostly distributed in western China.

Carbonate type deposits tend to occur in older, banded or lenticular, thin- to moderately thick-bedded strata of magnesium-rich dolomitic rocks, dolomite and marble. Hydrothermal ore fluids were derived from Hercynian and Yanshanian acid magmas and basic magmas. The orebodies are controlled by interlayer fissures and their shape and attitude are consistent with those of their country rocks. The distance of the deposits from the acid intrusions is 200 to 1,000 m, not exceeding 10 km at most; whereas that from the basic intrusions is only 1 m to a few dozen meters. Mineralization mainly occurs in the axial zones of synclines, cores of anticlines and interlayer joints and fissures. Generally strong serpentinization is favorable for mineralization. Fibers are commonly>8 mm long in simple or multiple veins and short in parallel veinlets or veins in which partings are developed. Deposits of this type are small in size with low-grade asbestos ore. However, the fibers occurring in some easily accessible and economically developed areas of North China and Northeast China are good in quality and of wide application. Therefore, both carbonate type asbestos and ultrabasic type asbestos are the main objects for China's asbestos production and development.

China's asbestos ore grade (fiber content) is generally high. According to the incomplete statistics, the ore grade is commonly 4%~5% or so. The grade of the ultrabasic type ore is relatively high, generally with an average grade of 2%~8%, e.g. it is 8.52% in the No. II mine of the Xiaobabao mine, Qilian County, Qinghai Province. The grade of the carbonate type ore is relatively low, generally with an average grade of 1%~3%, e.g. it is 3.2% in the Laiyuan deposit, Hebei. Asbestos of China is dominantly short-fiber one, mostly of grades VI and VII, and asbestos fibers at and better than grade III only account for 10.5% of the total reserves. For example, the fibers at and better than grade ¢÷ in the Shimian mine, Sichuan, make up 11.36% of the total reserves of the whole asbestos district and fibers of grades VI and VII 64.9%, of which fibers of grade III take up 42.07%.

The asbestos produced by the Xiaobabao mine, Qilian County, Qinghai Province is almost all short-fiber one, with grades VI and VII asbestos accounting for 94.25% of the total reserves of the whole mineral district and grades AA and I only 1.44%, but all the fibers soft, possess wet-spinning characteristics, good cleavability and a high crystallinity with its chemical composition approximate to the theoretical values, and contain few impurities which are easy to separate from the fibers. The asbestos fibers in the Mangnai deposit, Qinghai, have a very similar shape and crystallity to those of Qilian, but as appreciable Fe3+ in the composition replaces Mg in the octahedra, the asbestos tube walls thicken and fibers become hard and brittle and are bound by impurities so as to be separated with difficulty; so the fibers are hard to use in the wet textile industry. Nevertheless, as the Mangnai asbestos has a high crystallinity and a high strength, it is also applied widely. The acicular asbestos in the Laiyuan mine, Hebei, is a good raw mineral material for the manufacture of cement. The asbestos fibers in Shimian County, Sichuan, contain an appreciable amount of impurities which are complex in occurrence and are entangled and bound with asbestos fibers so as not to be separated easily, but the ore has a relatively high proportion of long fibers and so has a certain prospect.

Crysotile ore of China is usually mined as an ore of asbestos alone. The Zhonggangban and Zhiyuan Shizishan deposits, Yiyang County, Jiangxi, and the Tanggoushan and Lindong deposits, Rongshui County, Guangxi, which are small associated asbestos deposits, occur in serpentinite ore districts. In the Shimian deposit, Sichuan, and the Da'an deposit, Shaanxi, brucite asbestos is associated with serpenitne asbestos, but its reserves are not calculated separately. It is estimated that its reserves account for 1% of the total. Generally the mineral associated with chrysotile include talc, magnesite, sepiolite, wollastonite, tremolite, serpentinite, marble, dolomite, albite, gems (jade) and chromite, as well as such metallic elements as platinum, palladium and nickel, but their contents are low and their integrated evaluation was limited during previous asbestos exploration; so they cannot be mined as by-products.

China's major asbestos mines are mostly distributed in inaccessible high mountains in western China, where natural conditions are poor and asbestos is difficult to mine. The conditions of mining and benefication operations vary from place to place. For the asbestos mines in Qinghai, Xinjiang, Gansu and Shaanxi of northwestern China, orebodies are generally buried at shallow depth and suited to be open-cut mined, the hydrogeological conditions in the asbestos districts are simple, and the ore is easy to dress. For the mines in the vicinity of Shimian County, Sichuan, underground mining is required in most cases and much clay is mixed in ore so that washing becomes difficult and the cost increases, but long-fiber asbestos has a high proportion.